In other words: What Makes Stainless Steel STAINLESS?

Stainless steel does not rust, or remains stainless, due to the combination of alloys that are present within that grade of stainless and the conditions in the environment in which that grade is being used.  Stainless steel contains the following elements: Fe, Mn, Si, Cr, C, and then depending on the grade, some Ni and or Mo. In non-periodic table speak the elements are Iron, Manganese, Silicon, Chromium, Carbon, Nickel and Molybdenum.

These elements create a thin protective film which acts as a retardant to rust or stain. Stainless steel isn’t 100% stainless or rust proof in certain environments. The oxygen in the air as well as the water react with the elements in the steel to form a very thin, stable film. Chromium is the dominant leader in reacting with Oxygen to form this thin corrosion resistant film.  For a steel to be Stainless , by definition it must contain at least 10% Chromium.

The existence of this film creates a layer of resistance to corrosion by limiting the water or oxygen from penetrating that protective layer and gaining access to the metal beneath it. The film quickly and tightly around the surface. The layer is atomic in size, meaning it’s very small and imperceptible to the human eye. We can’t even see the corrosion (because it is there) it’s not visible because it’s on the atomic layer and humans would need instrumentation to see that type of corrosion and thus since we can’t see it the steel does appear stainless.

Rust on a stainless steel bar.

A stainless bar with surface issues shows rust. The bars above and below with no surface defects have no rust.

Standard “Steel” has a different reaction to the water molecules in oxygen and water in general. An unstable iron oxide film will not stop growing, unlike on a stainless bar where it stops at a molecular level and eventually will become visible due to the consistent corrosive environments wreaking havoc on the surface of that steel.

  Stainless Steel in Summary

To summarize, stainless steel will not rust because it reacts quickly and strongly enough to protect itself from further corrosion by forming a passive corrosion layer. Stainless is used in a wide array of products because of how durable and aesthetically appealing it is.

To the right is an image of rust on a stainless bar where some of the protective layer has been removed and thus is now prime real estate to rust. The bars above and below have not had any surface imperfections and thus no rust.

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